The invention relates to an electric discharge tube or discharge lamp comprising one or more low-temperature cathodes, to a flat-panel display screen comprising one or more low-temperature cathodes, and to a low-temperature cathode and a method of manufacturing said low-temperature cathode.
Conventional display screens comprise an electric discharge tube which operates according to the cathode ray tube principle. This means that electrons are accelerated from a cathode, then they are deflected and, finally, they impinge on a curved fluorescent screen. Such discharge tubes require a large construction depth or a strong curvature of the display screen, otherwise the difference in distance from the electron source to the front side of the display screen would cause the signs to be displayed in focus in the center yet distorted at the edge.
Also flat-panel display screens have been commercially available for a number of years. In the development of flat-panel display screens, various principles are in competition with each other. The present invention relates, inter alia, to flat-panel display screens having active systems, in which, unlike liquid-crystal display screens, the display screen is not operated by means of ambient light, but is self-luminescent. This type of flatpanel display screens includes plasma-display screens and flat-panel display tubes.
Flat-panel display screens were developed for three market segments, i.e. office automation, audio/video technology and navigation and entertainment. In the field of office automation, reference must be made, in particular, to mobile applications which range from the notebook computer, Personal Digital Assistant, fax machine to the mobile telephone. In the field of audio and video, the flat-panel display screens are intended not only for use in camcorders but also in television receivers and monitors. The third field of application comprises flat-panel display screens which are to be used as monitors for navigation systems in cars and aircraft, but also as displays for electronic games.
A drawback of the cathode ray tube is that it takes up a lot of space. This is caused by the fact that all electrodes originate from a single cathode and are brought, via deflection units, to the desired spot on the display screen, so that they are responsible for all pixels. The cathodes of these conventional cathode ray tubes are hot cathodes ("thermionic cathodes"). The generation of electron beams is based on thermionic emission. The conventional, heatable cathode is made, for example, of a small nickel tube on an end face of which there is provided an oxide layer, for example of barium oxide, which emits electrons very readily. The heating wire which is embedded in said nickel tube so as to be insulated, raises the temperature of the cathode to approximately 1200.degree. Kelvin (900.degree. C.), so that electrons are "evaporated out" of the oxide layer in the vacuum of the tube.
In the case of the flat display screens, however, the electrons are generated in a number of wire cathodes, flat-strip cathodes or in planar field emitters. Consequently, each cathode is responsible only for a small number of pixels.
For all types of flat-panel display screens, the manufacture of appropriate cathodes constitutes a key technology. A lot of effort has already been put into the development of cathodes and novel cathode materials suitable for the flat-panel display screen technology. The use of extended wire cathodes, flat-strip cathodes or planar cathodes in flat-panel display screens, instead of the conventional punctiform cathode, results in a much lower emission. Since the conventional hot cathodes have the disadvantage that the cathode must be maintained at a constant high temperature, which involves an additional thermal-energy consumption, one of the above-mentioned developments relates to the manufacture of cold cathodes. Hitherto, developments in the field of cold cathodes include, for example, microtip emitters or semiconductor emitters (AC-cathodes=avalanche cold cathodes). A drawback of said microtip emitter cathodes is the burning-out of single microtips and the high current noise of the individual microtips. AC-cathodes have the disadvantage that their emission is extremely localized, requiring a high positional accuracy of the cathodes.